In recent years, there has been demand for an electronic integrated circuit that can be driven at low voltage and that has high frequency and low noise. There has also been demand for a solid electrolytic capacitor that has low ESR and ESL. Metallic powder which is suitably employed in an anode electrode of a solid electrolytic capacitor may include, for example, powder of niobium, tantalum, titanium, tungsten, or molybdenum.
A tantalum capacitor, a typical capacitor which has small size, low ESR, and high capacitance, has rapidly become popular as a component in cellular phones and personal computers. Particularly, in a smoothing circuit of an exclusive power source of a microprocessor, the capacitor must have high capacitance (high CV value) and low ESR. The capacitance of a tantalum capacitor is effectively increased by increasing the surface area of fine metallic powder which is employed. Therefore, fine tantalum powder has been developed. At the present time, by means of a method in which potassium tantalum fluoride is thermally-reduced by use of sodium, the resultant primary powder is thermally-aggregated and deoxidized, and tantalum powder having a BET specific surface area of approximately 1 m2/g (mean primary particle size on the basis of specific surface area (d50)=400 nm) and a specific capacitance of 50,000 CV is produced in large amounts.
A niobium capacitor has long been studied as a solid electrolytic capacitor, since niobium oxide has a high dielectric constant and niobium is inexpensive compared with tantalum. However, the capacitor has not yet been employed in practice because of the low reliability of oxide film produced through anodizing. Namely, when niobium is oxidized through anodizing at high voltage, amorphous oxide film is crystallized, and thus leakage current increases and the capacitor is frequently broken.
However, recently, an electronic circuit has been driven at low voltage, and thus anodizing voltage can be lowered. In accordance with this trend, a niobium capacitor may be advantageously employed in practice, since niobium can maintain reliability at low anodizing voltage. Particularly, as a substitution for an aluminum electrolytic capacitor, a niobium capacitor, which has high capacitance and low ESR and ESL compared with an aluminum electrolytic capacitor, has become of interest for development.
In order to produce a niobium capacitor of high capacitance, niobium powder employed in the capacitor must have a mean primary particle size as reduced to BET specific surface area (d50) of 500 nm or less, preferably 400 nm or less, in the same manner as in the case of a tantalum powder. At the present time, known methods for producing fine niobium powder include a method in which potassium fluoniobate is reduced with sodium (U.S. Pat. No. 4,684,399); a method in which niobium pentachloride is reduced with hydrogen in a gas phase (Japanese Patent Application Laid-Open (kokai) No. 6-25701); and a method in which niobium powder of large specific surface area is obtained through grinding (WO 98/19811).
Of these methods, in a customary gas-phase method, super-fine niobium particles of mono-dispersion are obtained, and thus when a porous sintered body is formed and the compact is oxidized through anodizing, a neck portion is insulated, i.e., the neck is broken. Namely, in a gas-phase method, niobium powder which is suitably employed in an anode electrode cannot be obtained. In a grinding method, niobium powder is easily obtained at high efficiency, but the shape of the particles is irregular and the particle size distribution becomes broad, and thus the particles cause problems when they are employed in an anode electrode.
Therefore, in order to produce niobium powder particles of chain-type which are suitably employed in an anode electrode and which exhibit a sharp peak in the particle size distribution of the primary particles, a liquid-phase method such as a method for reducing a molten potassium fluoride salt with sodium or a method for reducing niobium chloride with molten metal is considered to be preferable.
When such fine niobium or tantalum powder is employed for producing an anode electrode of high capacitance, crystalline oxide tends to form during thermal treatment or oxidation through anodizing, and thus leakage current may increase. This is because when the surface area of the powder increases, the amount of oxygen in the powder also increases. Incidentally, anodizing voltage for forming a dielectric oxide film is lowered in accordance with reduction in rated voltage of a capacitor. Therefore, the formed dielectric oxide film tends to become thin, and the film has poor long-term reliability in spite of high capacitance.
In view of the foregoing, in order to suppress the effect of oxygen and enhance reliability of a thin film, a sintered body or a dielectric oxide film is doped with nitrogen after production thereof.
For example, U.S. Pat. No. 5,448,447 discloses a method in which an oxide film produced through anodizing is doped with nitrogen in order to lower the leakage current of the film and to enhance the stability and reliability of the film at high temperature. WO 98/37249 discloses a method for doping tantalum powder of high capacitance uniformly with nitrogen, in which ammonium chloride is added to reduced tantalum powder and nitrogen is introduced into the mixture simultaneously with thermal agglomeration of the mixture.
Other techniques include reduction in leakage current of an Nb—O film which is produced through sputtering of niobium, by doping the film with nitrogen (K. Sasaki et al., Thin Solid Films, 74 (1980) 83-88); and improvement of leakage current of an anode by employing a niobium nitride sintered body as the anode (WO 98/38600).
Japanese Patent Application Laid-Open (kokai) No. 8-239207 discloses a heating nitridation method in which tantalum or niobium powder which is obtained through reduction is heated in an atmosphere of nitrogen-containing gas, during thermal agglomeration or deoxidization.
In customary methods, nitridation proceeds on the surface of powder or film, and thus diffusion of nitrogen determines the rate of nitridation. As a result, the surface tends to be nitrided non-uniformly.
When the amount of nitrogen in a metallic powder is in excess of 3,000 ppm, for example, in the case of tantalum powder, crystalline nitride such as TaN0.04, TaN0.1, or Ta2N is formed. When the powder is further doped with nitrogen, a crystal phase predominantly containing TaN or Ta2N is formed. When powder containing such a crystalline nitride is employed, the specific capacitance of an anode electrode is lowered and reliability of a dielectric film is reduced.
Meanwhile, when a sintered body or a dielectric oxide film is doped with nitrogen after production thereof, a nitridation process is additionally required, which results in poor productivity.
Briefly, there has not yet been found a nitrogen-containing metallic compound in which fine niobium or tantalum is uniformly doped with a sufficient amount of nitrogen, a crystalline nitride compound is not formed, and nitrogen is contained to form a solid solution within the metallic crystal lattice.